Imidazole → imidazolidine. Preparation by reduction of [Os(H)(CO)(PPh3)2(PyaiR)]+/0[Os(H)(CO)(PPh3)2(PyaiR)]+/0 with NaBH4 and characterisation of the products (PyaiR = 1-alkyl-2-{3′-(pyridylazo)}imidazole)

The reaction of 2-(3′-pyridylazo)imidazole (PyaiH) or 1-alkyl-2-(3′-pyridylazo)imidazole (PyaiR) with [Os(H)(Cl)(CO)(PPh3)3][Os(H)(Cl)(CO)(PPh3)3] in THF or MeCN has synthesized stable red coloured [Os(H)(CO)(PPh3)2(Pyai)][Os(H)(CO)(PPh3)2(Pyai)] (2a) or [Os(H)(CO)(PPh3)2(PyaiR)](PF6)[Os(H)(CO)(PPh3)2(PyaiR)](PF6) (2b–2d). While the reaction of PyaiR and [Os(H)(Cl)(CO)(PPh3)3][Os(H)(Cl)(CO)(PPh3)3] in dry n-heptane has separated shining green compound and has been identified azo-anion radical, [Os(Cl)(CO)(PPh3)2(PyaiR−)] (4b–4d). The reaction performance in n-hexane or toluene is very poor. Upon addition of Cl2 in MeCN to CH2Cl2 solution of 2 or 4 the colour has changed sharply to red-violet and has isolated [Os(Cl)(CO)(PPh3)2(PyaiR)]+[Os(Cl)(CO)(PPh3)2(PyaiR)]+ (3). The reaction of 2 in ethanol with NaBH4 suspension under dinitrogen has reduced coordinated imidazole group to imidazolidine (5). One of the structures of azo-imidazolidinate-hydrido-osmium–carbonyl, [Os(H)(CO)(PPh3)2(PyaiH2)] (5a), is characterized by X-ray diffraction study. Other spectroscopic studies (IR, UV–Vis, NMR) are used to identify the structure of all the complexes. Electrochemistry shows two consecutive anodic peaks (Epa) and suggest irreversible oxidations Os(III)/Os(II), Os(IV)/Os(III) along with azo reductions.

Graphical abstractReduction of [Os(H)(CO)(PPh3)2(PyaiR)](PF6) (2) with NaBH4 has separated [Os(H)(CO)(PPh3)2(PyaiR′H2)]+/0[Os(H)(CO)(PPh3)2(PyaiR′H2)]+/0 (5), an imidazole (C(4)–C(5)) reduced product. While the reaction of PyaiR and [Os(H)(Cl)(CO)(PPh3)3] in dry n-heptane has separated shining green compound and has been identified azo-anion radical, [Os(Cl)(CO)(PPh3)2(PyaiR−)] (4b–4d).Figure optionsDownload full-size imageDownload as PowerPoint slide